As @quantj said the only SketchUp ‘join’ tool is inside LayOut…
BUT if you mean the tool which can ‘join’ two lines into a single ‘curve’ [aka ‘polyline’] then that is usually called ‘weld’.
There’s the original ‘weld’ tool from RickWilson and more recently several ‘improved’ versions - like my own http://sketchucation.com/pluginstore?pln=TIG-weld